The manufacturing and assembly of electronic circuit boards is a complex and multistep process resulting in products of considerable and well-known utility. In general, a circuit is superimposed on a metal surface by patterning areas of conductivity and resistance. This is done using a suitable photoresist coating as protection and etching unprotected areas. The resultant is a set of metal layers on the board. As part of this process, in addition, certain features of this process involve procedures which utilize protection of areas, which are not to be affected, using adhesive tape. After completion of these processes, the tape is removed, and the adhesive cleansed from the board. The adhesives subject to cleaning are generally silicone or vinyl polymers.
After the unassembled boards have been fabricated, the components which will serve as gates, resistors, capacitors, etc. are soldered to the fabricated board. The soldering process, which results in a board assembled with its components, uses fluxes to insure good bonding between the metal of the fabricated board and the metal contact of the component through the interdigitation of the solder into each metal surface. To achieve this, the solder must be capable of wetting the surface and there can be no impurities impeding the integration. The flux, therefore, is used to degrease and decontaminate the surface to allow free access of the solder. Finally, the remainder of the flux must be cleansed from the assembled board.
The invention provides a superior composition for removing the flux residues in an efficient and thorough manner. The composition contains, in admixture, a terpene or terpenol, a surfactant, and an aprotic polar organic solvent. The admixture may be dispersed in water for use, depending on the nature of the flux to be removed.
Compositions of these substances have been sold for removal of adhesive residues at the above-mentioned, different, earlier stage of the process. However, the chemical and physical nature of the tape residues are very different from the flux compositions, and as far as Applicants are aware, these compositions have not heretofore been used for flux removal.
The fluxes which are subject to removal by the compositions of the invention are those customarily used in circuit board manufacture. These are of two major types: synthetic water soluble and rosin-containing fluxes. Rosin is a by-product of the distillation of pine tree sap and contains about 90% rosin acids and 10% neutral material, both of which are basically tricyclic isoprenoid condensates. In the operation of this flux, the rosin acids react with the surface metal oxide layer and do not attack the freshly exposed metal. The rosin-containing fluxes may also be activated using organic amines, acids, amides or halogen-containing materials and are thus used to produce rosin mildly activated (RMA) or rosin-activated (RA) fluxes. Most of the activating substances, apparently, decompose to liberate hydrochloric acid so that metal chloride salts formed from reaction with the clean metal surface become suspended in the flux.
The synthetic or water-soluble fluxes are also used in soldering electronics assemblies; the active components are usually amine hydrochlorides or hydrobromides along with water-soluble organic acids. These activators are dissolved in a solvent such as a glycol ether, alcohol, or their blends with water. Occasionally these fluxes can be washed away simply by using water; however, because they are so reactive, their use in electronics components may be disadvantageous. These fluxes can also be cleaned away usefully using the compositions of the invention, as their water solubility often is not maintained during the soldering process; while some of the residues remain water-soluble, others are not. For example, decomposition products caused by heating of the organic materials are often insoluble, and the chlorides of certain metals, for example lead chloride, may be tightly bonded to the substrate. Therefore, simple rinsing with water is often inadequate to remove the putatively "water-soluble" residues generated from these fluxes.
There is no dearth of compositions which have been tried for solder flux removal in the electronic board manufacturing process. For example, U.S. Pat. No. 4,640,719 describes compositions with a minimum of 60% terpene with the balance of the composition being a surfactant. A typical composition illustrated as the most preferred embodiment contains 90% limonene, and the 10% detergent component composed of 6.5% poly(7)ethoxynonyl phenol; 2.1% poly(10)ethoxynonyl phenol, and 1.4% sodium dioctyl sulfosuccinate. This composition is asserted to be capable of removing adhesives as well as flux; however, no data are given to support this asserted property.
U.S. Pat. No. 4,276,186 to Bakos et al describes a composition of at least 50% by weight of N-methyl-2-pyrrolidone and at least 5% by weight of a water-miscible alkanolamine, with the balance of 0-35% of a nonpolar hydrocarbon or halogenated hydrocarbon solvent. This composition is said to be useful in cleaning solder flux or flux residues from substrates. The cleaning process evidently is advanced by relatively high temperatures.
The Printed Circuits Handbook Koombs, C. F. ed., on pages 16-2 to 16-13 describes methods and compositions used for cleaning soldered circuit boards and freeing them of flux residues. This description shows the effect of various single component solvents such as perchloroethylene and various fluorocarbons to dissolve fluxes. Other alternatives include saponification of the acidic components in rosin-based solvents, and the use of water, although in large quantities with incomplete results, to remove the residues of putatively water-soluble fluxes.